(1) Field of the Invention
This invention relates to an optical module, an optical transceiver, an optical transmission system and a fabrication method for an optical module, suitable for use with a multi-channel optical transceiver (for example, a wavelength division multiplexing multi-channel optical transceiver) which includes a planar optical device such as, for example, a planar light emitting laser or a photo-diode (photo-detector).
(2) Description of the Related Art
In a case wherein an optical module such as, for example, a multi-channel optical transceiver uses a planar optical device such as a planar light emitting laser or photo-diode, since a light incidence face or a light emitting face of the planar optical element extends in parallel to a mounting board, light is incident or emitted perpendicularly upon or from the mounting board.
Meanwhile, in such an optical module as mentioned above, it is necessary to achieve reduction in size and thickness.
In order to achieve reduction in size and thickness, preferably optical fibers (optical fiber array) are disposed in parallel to a mounting board. In this instance, end faces of the optical fiber and the light incidence face or light emitting face of the planar optical device have a relationship of the substantially right angle to each other. Therefore, such various proposals as described below have been made in order to curve the paths (light paths) of incidence or emitting light perpendicularly to the light incidence face or light emitting face of a planar optical device mounted on a board by approximately 90 degrees to optically connect the optical fibers and the planar optical device to each other.
For example, Japanese Patent laid-Open No. 2005-115346 discloses a technique which uses an optical waveguide structure of a three-dimensional configuration wherein optical waveguides are formed on a curved plane for curving the advancing direction of light such that light to be incident to or emitted from a planar optical device is guided along the curved plane and coupled to an optical fiber array (refer to, for example, FIGS. 27 and 28).
Meanwhile, Japanese Patent Laid-Open No. 2003-322740 discloses that, in order to optically couple optical waveguides provided along a mounting face and a planar light emitting laser to each other, it is necessary to convert the direction of light by 90° and, as a method therefor, a 45° mirror is formed as a direction converter on a waveguide film which connects devices to each other.
Incidentally, as one of methods of making it possible to expand the transmission band width, an attempt to introduce a wavelength division multiplexing technique to increase the transmission capacity per one channel is available, and it is disclosed that a multiplexer or a demultiplexer which uses spatial propagation of light and a reflecting optical system is provided between a planar light emitting laser or a photo-diode and optical fibers (for example, refer to Brian E. Lemoff et al., “MAUI: Enabling Fiber-to-the-Processor with Parallel Multiwavelength Optical Interconnects”, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 22, NO. 9, September, 2004).
Incidentally, in an optical module such as a multi-channel optical transceiver as described above, it is a subject to reduce the cost for a portion (optical coupling portion) which optically interconnects a planar optical device [planar optical device array; for example, a VCSEL (Vertical-Cavity Surface-Emitting Laser) array or a PD (Photo Detector) array] and optical fibers (optical fiber array).
Therefore, also common use of parts has been and is proceeding gradually. At present, both of the pitch (array pitch) between a plurality of planar optical devices which form a planar optical device array and the pitch (array pitch) between a plurality of optical fibers which form an optical fiber array have been standardized to 0.25 mm. Further, both of the number (array number) of planar optical devices which form a planar optical device array and the number (array number) of optical fibers which form an optical fiber array have been standardized to 4, 8, 12 and 24.
Meanwhile, for example, in the case of a multi-channel optical transceiver, a VCSEL array and a PD array are mounted in a juxtaposed relationship with each other on a board. However, a mounting gap (gap between chips) of, for example, approximately 1 mm is essentially required between the arrays.
In this instance, also between optical fibers (VCSEL fibers) optically connected to the VCSEL array and optical fibers (PD fibers) optically connected to the PD array, a gap corresponding to the mounting gap is required.
Once the pitch is standardized to 0.25 mm between a VCSEL array or a PD array and an optical fiber array and also the array number is standardized to 4, 8, 12 or 24 in such a manner as described above, in order, for example, for a multi-channel optical transceiver of 8 channels (4-channel input+4-channel output) to be ready for the standardization, such a countermeasure is taken as to adopt a standardized optical fiber array for 12 channels by adding 4 channels corresponding to the gap (for example, 1 mm) between the chips to 8 channels originally required for the input and the output.
In this instance, optical fibers for the four channels corresponding to the gap between the chips are useless because they are not used for the input or the output. Further, since an optical fiber array for 12 channels is inferior in all aspects such as the performance and the price to an optical fiber array for 8 channels, preferably an optical fiber array for 8 channels is used.
Therefore, it is desired to implement a simple and convenient structure which can optically connect planar optical devices (planar optical device array) such as planar light emitting devices or planar photo-detectors and optical fibers (optical fiber array) to each other without providing a gap, which corresponds to a gap (mounting gap) between the chips, between optical fibers optically connected to the planar light emitting devices (VCSEL array) and optical fibers optically connected to the planar photo-detectors (PD array).
It is to be noted that this problem is solved in principle if it is possible to produce a part which connects to an optical fiber array for 8 channels with the waveguide distance reduced midway from the planar optical devices to the optical fibers.
However, where an optical waveguide structure of a three-dimensional configuration wherein optical waveguides are formed on a curved plane for curving the advancing direction of light is used as disclosed, for example, in Japanese Patent Laid-Open 2005-115346, it is necessary to form optical waveguides of a high degree of accuracy for optically connecting optical devices and optical fibers to each other on a curved plane. However, at present, a countermeasure which can implement such a complicated highly accurate three-dimensional structure as described above simply and conveniently is not available and has not been placed into practical use. Further, it is difficult from restrictions in a fabrication method to implement a structure which can simultaneously achieve also pitch conversion.
Meanwhile, another countermeasure wherein a mirror (90° deflecting mirror) is used to achieve high-density optical connection between chips and pitch conversion as disclosed, for example, in Japanese Patent Laid-Open No. 2003-322740, is disadvantageous in that the loss at the mirror (that is, the absorption loss on the surface of the mirror) is significant and the optical connection and the pitch conversion are liable to be influenced by the accuracy in alignment and the accuracy in working. Further, crosstalk at an intersecting location between waveguides cannot be avoided, and the optical performance is not high.
Incidentally, although the aforementioned thesis “MAUI: Enabling Fiber-to-the-Processor with Parallel Multiwavelength Optical Interconnects” discloses a high-grade module which introduces a wavelength division multiplexing technique and uses spatial propagation of light and a reflecting optical system, the module has the following problems.
First, since a spatial propagation system is used, the beam diameter cannot be made very small (for example, a diameter of 250 μm), there is a limitation to miniaturization of the module. Further, since a spatial multiple reflecting optical system is adopted, alignment is difficult. Furthermore, it is difficult to assure a high impact resistance of the product. As a result, also reduction in cost is difficult.